This invention is concerned with the formation of fine-grained polygonized halide bodies. In particular, the present invention is concerned with the preparation of high strength halide bodies for use as optical components in infrared systems.
One of the more critical problems encountered in the development of high power infrared lasers is the development of laser windows which are highly transparent to laser radiation at 10.6 microns and at 3 to 5 microns. At the present time, considerable research effort has been devoted to the development of laser windows from the co-called covalent compounds consisting typically of II-VI compounds such as cadmium telluride, zinc telluride and zinc selenide. The need for improved laser window materials, however, is well known. F. Horrigan et al, "Windows for High Power Lasers," Microwaves, Page 68 (January, 1969); M. Sparks, "Optical Distortion by Heated Windows in High Power Laser System," J. Appl. Phys., 42, 5029 (1971).
The need for improved laser windows is based on the extremely high laser power throughput required and the fact that the laser windows constitute structural members. In order to maintain high throughput and minimize adverse effects, the amount of energy transferred to the window must be kept low. Laser beam energy can be transferred to the window in two ways: heating of the window caused by either bulk or surface absorption of the beam, or direct conversion of the beam energy to mechanical energy by Brillouin scattering or electrostriction. This energy transfer produces several undesirable effects such as lensing and birefringence, which result in degradation of beam quality and polarization. In extreme cases, severe thermal stresses can be produced in the windows. These stresses, which are further aggravated by the fact that the windows are mounted in a cooling clamp, may lead to fracture of the windows.
The low absorption coefficients of the halides make them outstanding candidates for optical components in infrared systems. The alkali halides exhibit low absorption at 10.6 microns, and the alkaline earth halides exhibit low absorption in the 2 to 6 micron region. Furthermore, because the temperature coefficient of the index of refraction and the thermal expansion have opposite signs, the two effects tend to compensate optical path changes due to temperature, making these materials useful in applications in which heating by a laser beam is anticipated. Halide crystals, however, have low yield strengths and are highly susceptible to plastic deformation. J. J. Gilman, "Plastic Anisotropy of LiF and Other Rock Salt Type Crystals, " Acta Met., 7, 608 (1959). These mechanical properties of single crystal alkali halides have precluded their use as high power laser windows.
The outstanding transparency of the halide materials makes it very attractive to attempt to overcome their mechanical deficiencies. Halides can be strengthened without altering their optical properties by hot working of single crystals to produce fully dense polycrystalline material. R. J. Stokes andd C. H. Li, Materials Science Research, Vol. 1, pages 133-157, edited by H. H. Stadelmaier and W. W. Austin, Plenum Press, New York, 1963; N. S. Stoloff et al, "Effect of Temperature on the Deformation of KCl-KBr Alloys," J. Appl. Phys., 34, 3315 (1963); and R. J. Stokes, "Mechanical Properties of Polycrystalline Ceramics," Proc. Brit. Ceram. Soc., 189 (1966). This technique involves the deformation and recrystallization of crystals at elevated temperatures to introduce grain boundaries, thereby producing polycrystalline halide materials. The techniques described in these articles involved the extrusion of halide materials.
Fine-grained polygonized halide bodies may also be produced by pressing, rolling, or a combination of pressing and rolling. The previously mentioned patent application by R. H. Anderson describes a process for forming fine-grained halide bodies at low temperatures by use of a constraint technique. This technique yields structures which are extremely fine-grained and which can exhibit yield strengths over an order of magnitude higher than the starting single crystal billet. A further advantage of this process is that under certain conditions (temperature, strain rate, initial crystal orientation) the optical properties of the polygonized billet are identical to those of single crystal material.
One problem which has been discovered in fine-grained polygonized halide billets formed by hot working in a rapid and extensive grain growth at room temperature. This behavior occurs in both unconstrained and constrained billets which are hot worked at low temperatures. This room temperature grain growth is undesirable, since polygonized halides having a small grain size exhibit higher strength than halides having larger grain size.